test_segmentation.py fails if every test output is background-only

[anandthakker]

This line fails on a divide-by-zero error:

skynet-train/segnet/test_segmentation.py

Line 96 in a21a304

'correctness': pixels_correct / pixels_predicted_fg,

This is an extreme edge case, though, which probably indicates that something is wrong with the weights being used.